Engine exhaust catalysts require periodic regeneration to restore catalytic activity and reduce catalyst oxidation. For example, catalysts may be regenerated by rich injecting fuel to reduce the amount of oxygen stored at the catalyst. Catalyst regeneration strategies may vary for different engine systems.
One example approach for exhaust catalyst regeneration in a variable displacement engine (VDE) system is described by Doering et al. in U.S. Pat. No. 7,044,885. Therein, the engine includes two cylinder groups, each coupled to respective catalysts. An engine control system is configured to alternate the cylinder group that is selectively disabled during a VDE mode of operation to balance engine wear and catalyst usage.
However, the inventors herein have identified a potential issue with such an approach. If the catalyst on one of the cylinder groups is partially regenerated and the other cylinder group is selected for disablement (based on the alternating pattern), fueling may be continued to the partially regenerated catalyst, to complete regeneration, before the cylinders of that engine can be operated. Consequently, additional fuel may be consumed to complete the regeneration. As such, this results in degraded fuel economy.
Thus, in one example, the above issue may be at least partly addressed by a method of operating an engine including a first group of cylinders coupled to a first catalyst and a second group of cylinders coupled to a second catalyst. In one embodiment, the method comprises, selectively deactivating either the first or the second group of cylinders, the selective deactivation based on a regeneration state of the first catalyst relative to the second catalyst. In this way, by selecting the group of cylinders with the less regenerated catalyst, additional fuel may not be wasted in completing catalyst regeneration.
For example, a variable displacement engine (VDE) may be configured with a first and a second group of cylinders, each cylinder including a selectively deactivatable fuel injector. In response to a cylinder deactivation request (such as, at low engine loads), an engine control system may compare the regeneration state of a first catalyst, coupled to the first group of cylinders, relative to a second catalyst, coupled to the second group of cylinders. If the first catalyst is less regenerated than the second catalyst, the first catalyst may be selected for deactivation. In comparison, if the second catalyst is less regenerated than the first catalyst, the second catalyst may be selected for deactivation. In another embodiment, the selection may be further based on a catalyst temperature. As such, if both the catalysts are sufficiently regenerated, the control system may select a cylinder group based on the deactivation order. Thus, if the first group of cylinders was deactivated in the previous VDE cycle, the second group of cylinders may be selected for deactivation on the current cycle, and vice versa. By alternating cylinder group deactivation when both catalysts are substantially regenerated, engine wear and tear may be balanced between the two groups of cylinders.
In this way, by selecting a group of cylinders based on the regeneration state of exhaust catalysts, fuel wastage can be reduced. Specifically, by selectively deactivating a group of cylinders that is coupled to a partially regenerated catalyst, even if the same group of cylinders was previously deactivated in a preceding VDE cycle, fuel loss related to complete regeneration of a catalyst (on a cylinder group that is subsequently deactivated), can be pre-empted. In this way, fuel economy may be improved without substantially affecting engine or catalyst wear.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.